In a circular dividing table for a machine tool, the shaft connected with a circular table and driven by a drive motor must be aligned precisely at a desired angular position and then be clamped by a clamping device. In this regard, an apparatus is known in which a DD(direct drive) motor is used as the drive motor and an electromagnetic brake is provided in place of a friction brake as the clamping device(see Japanese Laid-Open Patent Application 2007-125640, for example).
A friction brake of a clamping device is composed of, for example, a brake disc secured integrally with a shaft and clamping members to be pressed against the brake disc. In this, a circular dividing table is known in which a clamping member driven by a piston actuated with air pressure is pressed against the brake disc and the circular dividing table is aligned at the predetermined angular positions (see Japanese Patent No. 3713190, for example). In the circular dividing table disclosed in the above mentioned patent, clamping members on the movable part are driven and pressed against the brake disc to clamp it, after which a return spring acts so as to release the clamping members and return them to the original position.
However, said arrangement is not preferable in which the clamping member is released and returned to the original position by a return spring. This is because of the fact that the action of the arrangement becomes unstable due to the deterioration of the return springs caused by metal fatigue etc. or the retardation of the action may occur due to troubles in mechanism such as poor lubrication. In this regard, some measures are taken such as to use a return spring of a larger spring constant, but such measures suffer from a limitation in the view point of machine design.